1. Field of the Invention
The present invention relates to code-division multiple access (CDMA) mobile communications networks.
2. Description of the Related Art
In a CDMA mobile communications network, multi-user interference reduces the bit error rate (BER) performance of the radio link and as a result reduces the network capacity. Such performance degradation is exacerbated by imperfect power control and high-rate data users. In the proposed wideband CDMA (W-CDMA) element of the standard referred to as UMTS/IMT2000 the multi-user interference caused by high-rate data users to low-rate service users is expected to be significant. UMTS relates to the Universal Mobile Telecommunications System standard of the European Telecommunications Standards Institute (ETSI) and IMT2000 relates to the International Mobile Telecommunications 2000 standard of the International Telecommunications Union (ITU-R) TG8/1.
Incidentally, in the proposed W-CDMA standard, a high data rate would be 384 kbit/s or 2 Mbit/s, for example. In comparison, a digitised voice signal typically has a low bit rate of, say, 20 kbit/s.
Heretofore, many multi-user interference cancellation techniques have been proposed with a view to reducing and potentially eliminating the performance degradation caused by multi-user interference. However, in practice the complexity of such previously-proposed techniques makes them only suitable for implementation in base stations of a CDMA network. This, however, only enables the performance degradation caused by multi-user interference in the uplink direction (from the mobile stations to the base station) to be addressed.
In UMTS/IMT2000 it is recognised that many services will demand an asymmetric capacity with relatively high bit rates in the downlink direction and relatively low bit rates in the uplink direction, for example Internet web-browsing. For such services having a high bit rate in the downlink direction, multi-user interference cancellation techniques implemented in the base stations are of relatively little value as the uplink-direction bit rates are low.
It is therefore desirable to provide a low-complexity multi-user interference cancellation technique suitable for implementation in a mobile station of the network. Such a technique can increase the effective network capacity in the downlink direction, particularly when the network is providing services to high-rate data users in the downlink direction.